1. Field of the Invention
The present invention relates to a radiation image processing technique.
2. Description of the Related Art
When making observations of a subject (human body in particular) by detecting distribution of transmitted radiation such as X-rays that are transmitted through the subject, radiation imaging systems utilizing X-ray TV systems and accumulative fluorescent body (photostimulable phosphor) are used. In recent years, capturing of radiation images of a subject by directly receiving radiation that has been transmitted through a subject with a flat panel X-ray sensor, a large-scale image sensor (solid-state image sensor) that utilizes semiconductors, has become widespread.
When radiation passes through a subject, a portion of the radiation is scattered, and the contrast of the obtained image drops due to this scattering. In order to eliminate this scattered radiation, a member called anti-scattering grid (also referred to as simply “grid” hereinafter) is placed between the subject and the image sensor. This grid is an arrangement of radiation absorptive members, such as lead plates having thickness of less than approximately 1 mm, in grid or row formation, and reduces scattered radiation which arrives at the image sensor by absorbing scattered radiation advancing in random directions. However, even when this anti-scattering grid is utilized, a large amount of scattered radiation which is scattered from the subject reaches the image sensor, and causes deterioration of image contrast. For this reason, there is a need to further reduce the effect of scattered radiation. The present invention relates to a method of reducing scattered radiation.
Conventionally, a method of obtaining a radiation image is known in which the effect of scattered radiation is reduced by scattered radiation distribution obtained by utilizing the drop in contrast from the scattered radiation of the subject by an anti-scattering grid. More specifically, when an anti-scattering grid is used, its shade appears as stripes (also referred to as “grid image” hereinafter) in the image obtained by the image sensor. This grid-image appears as prominent shades when there is no scattered radiation, and becomes obscure when there is scattered radiation. Using the difference, it is possible to obtain a two-dimensional distribution of scattered radiation. A technique resembling this is disclosed in Japanese Patent Laid-Open No. S62-092662.
However, as in Japanese Patent Laid-Open No. S62-092662, it is not possible to accurately retrieve an amount corresponding to the scattered radiation merely by subtraction between pixels above which the grid exists. This is due to the fact that the pixel value of a pixel above which the grid exists is determined according to either equation (1) or equation (2) when there is no subject and when there is a subject, respectively.Pixel value=irradiation component−grid component  (1)Pixel value=irradiation component−subject component+scattered component  (2)
In the above equations, the pixel value on the left and each of the items on the right are values from logarithmic conversion of the X-ray amount that reaches the image sensor. The irradiation component is the pixel value that reaches the image sensor when there is no subject. The grid component is the pixel value of the grid shade, and the subject component is the pixel value of the subject shade. The scattered component is the pixel value corresponding to the amount of scattered radiation scattered from the subject. From the subtraction of equations (1) and (2), an amount [scattered component−subject component] can be obtained. However, this amount not only includes the scattered component, but also includes the subject component, and from this fact it is clear that it is not possible to obtain only the amount corresponding to the scattered radiation from the subtraction between pixels above which the grid exists. In other words, with the method disclosed in Japanese Patent Laid-Open No. S62-092662, it is not possible to gauge the amount of scattered radiation. In order to determine whether there is a large amount of scattered radiation, it is necessary to eliminate the effect related to the subject component.